The Global Positioning System (GPS) is a constellation of satellites operated by the United States government that provides microwave navigation signals to an unlimited number of users. Several methods for interpreting the navigation signals have been used to extract user's location in world or local coordinates with varying degrees of accuracy. Position measurements between two local receivers can be made to meter-level accuracy using GPS differential code phase measurements, and further, centimeter-level accuracy can be achieved between two local receivers using GPS differential carrier phase measurements. Unfortunately, establishing proper carrier phase measurements requires one of several initialization procedures to be performed, any of which depend on the geometric layout of the overall system and motion on the part of either the satellites or the mobile receiver. The initialization process is known as carrier cycle ambiguity resolution or integer resolution and can require that the user wait a few to tens of minutes for the satellites to subtend an appreciable arc along their orbits, or it can require that the user move past a fixed pseudolite transmitter. For some applications, the initialization process is a tolerable inconvenience, for many more general applications the initialization process causes a significant loss of functionality and is unacceptable. This is especially true for applications that depend on very high accuracy, high integrity, and fast, continuous update rate of the position information, which is the case for most heavy machinery control applications.
Pseudolites, or pseudo satellites, have been proposed for use in GPS applications where satellite coverage alone is insufficient to generate a position solution, and therefore some means of augmentation is needed. Also, pseudolites have been used to provide geometric advantage to the positioning system as a whole, such that cycle ambiguities can be resolved in a consistent and reliable manner if the user passes close to the pseudolite. The term “pseudolite” originated in the early design of GPS and even pre-dates the launch of GPS satellites but has been used as an appellation for a variety of devices. A common perception of a pseudolite, though, is that of a ground-based or local transmitter that, from the user's point of view, may be indistinguishably substituted one-for-one for a GPS satellite. The idea of using a pseudolite to facilitate rapid cycle ambiguity resolution is a more recent concept and is a function that arises because of real differences between close-range pseudolites and GPS satellites. Most pseudolite devices that have been built or suggested attempt to adhere to the ideal equivalence between satellites and pseudolites and therefore minimize deviation from the GPS signal plan. This is done hoping that existing GPS receivers may use pseudolites without hardware modification but sacrificing performance.
Several techniques have been used to attain centimeter-level navigational accuracy from GPS and are well known in the art. Each of these techniques uses reconstructed carrier signal(s) to gain finer precision than available from code phase measurements alone. Because the pure carrier signal is ambiguous with respect to carrier wavelengths, special algorithms are used to resolve the ambiguity—a process referred to as “solving for the integers” or “cycle ambiguity resolution”. These techniques rely on either satellite motion, motion with respect to pseudolites, or multiple satellite systems (GPS and GLONASS) to resolve cycle ambiguities. In all cases, a reference receiver at a known surveyed location with a data link connecting it to the roaming receiver is required to obtain acceptably rapid convergence to the correct set of integers.
Satellite orbital motion provides geometric diversity of carrier phase measurements when taken by a receiver over a few minutes or more of time. The technique is commonly referred to as Real-Time Kinematic (RTK) GPS. A receiver tracking a single frequency, L1, or both L1 and L2 can employ the technique. When five or more satellites are tracked continuously over a period of time, the code-phase position solution contains at least one redundant measurement. This extra information may be accumulated to eventually overwhelm the number of unknown cycle ambiguities and thus converge on the carrier phase integers. The process works if L1 carrier is tracked alone, or if L1 and L2 carriers are tracked simultaneously. The dual frequency approach converges faster because the L1 and L2 signals are broadcast from a common clock and the frequency separation between L1 (1575.42 MHz) and L2 (1227.6 MHz) provides advantageous observability over L1 alone. Convergence typically takes a minute or more when both L1 and L2 are employed, and may take ten minutes or more when L1 alone is employed.
A similar approach uses both GPS and GLONASS satellites. By using GLONASS satellite signals in addition to GPS signals in the code phase solution, up to twice as many signals can be employed. This increases the number of redundant measurements available to overwhelm the unknown cycle ambiguities. The additional number of satellites increases overall system availability, but the separation between GPS L1 and GLONASS frequencies (1610.6-1613.8 MHz) is much smaller than GPS L1/L2 separation and provides less advantage in terms of convergence time, which may be a few minutes or more.
A mobile receiver passing by one or more pseudolites incurs geometric diversity of its carrier phase measurements in a manner somewhat reciprocal to satellite motion in the RTK technique. In this case, continuous tracking of at least four satellites and one or more pre-surveyed pseudolites provide redundant measurements in the nominal code-phase solution. If significant geometry change occurs relative to the pseudolites, the extra information afforded by the pseudolites may be accumulated to eventually overwhelm the number of unknown cycle ambiguities and thus converge on the carrier phase integers. This method has been used for automatic landing of aircraft, whereby two pseudolites are placed on the ground on opposite sides of the approach path and integers are resolved as the plane flies between the pseudolites on final approach. The drawback of this method is that the vehicle must follow a nominal trajectory past the pseudolites, and the integers cannot be resolved until a significant amount of geometry change has occurred with respect to the pseudolites. In many applications, such as construction and open-pit mining, the vehicles may not move along convenient trajectories to make this technique practical. Again, carrier phase accuracy would not be available for significant periods of time.
The GPS signal structure, being based on an orbiting satellite constellation, has been formed around a very different set of requirements than a local pseudolite constellation would impose. Among these, the range-ratio for a local pseudolite system ought to be much greater than that of GPS, because it shall be desired to operate in close proximity to any one pseudolite in the system, as well as from several kilometers away—the full range over which differential carrier phase is feasible. If a user receiver is to operate as close as 10 m, and as far as 10 km from a pseudolite, the range ratio is 1000:1, which demands the receiver have a dynamic range in power of 60 dB.
The near-far ratio for conventional GPS receivers is firstly limited by the periodicity of the C/A code patterns. Because of the relatively short length, 1023 bits, of the C/A codes, their cross-correlation may exhibit signal to interference (S/I) of −21.6 dB relative to the peak correlation power. A typical GPS receiver requires a minimum S/I of 6 dB to track C/A code, leaving 15.6 dB of margin. The 15.6 dB margin is the maximum power that an interfering signal may be higher than a tracked signal, before disrupting the tracked signal. This 15.6 dB of margin translates to a 6:1 near/far range ratio, much smaller than desired for a pseudolite system. Subsequently, most conventional GPS receivers are designed to operate over only this limited dynamic range.
Accordingly, an object of this invention is to rapidly resolve integer ambiguity—even without significant vehicle motion relative to the pseudolites.
Another object of the invention is to enhance the integrity and speed of the pseudolite technique described above.
Still another object of the invention is to maintain full compatibility and non-interference with existing GNSS signals.
Still another object of the invention is to maintain a significant range of operation for the user receiver, especially allowing operation in close proximity to the pseudolite transmitters.
Still another object of the invention is to operate free of special government licensing.
Still another object of the invention is to leverage conventional GPS equipment, including GPS receivers and pseudolites, to reduce the cost of a system.
These and other goals of the invention will be readily apparent to one of skill in the art on reading the background above and the description below.
Pseudolite systems have been disclosed by Cohen et al., U.S. Pat. No. 5,572,218; Longaker, et al., U.S. Pat. No. 6,271,788; Sheynblat U.S. Pat. No. 5,646,630; Trimble, et al., U.S. Pat. No. 5,708,440 and U.S. Pat. No. 5,686,924; Janky, U.S. Pat. No. 6,198,432; Schellenberg et al., U.S. Pat. No. 5,886,666; Kyrtsos et al., U.S. Pat. No. 5,629,855; Farley et al., U.S. Pat. No. 6,336,076; Schneider et al., U.S. Pat. No. 6,300,898; Mickelson, U.S. Pat. No. 6,031,487; Beal, U.S. Pat. No. 6,101,178; and Gounon et al., U.S. Pat. No. 5,757,314.
U.S. Pat. No. 5,572,218, by Cohen et al., describes a pseudolite system for landing aircraft that relies on vehicle motion past a pair pseudolites on final approach to resolve carrier integer cycle ambiguities, and hence achieve precise position solutions.
U.S. Pat. Nos. 6,271,788 and 5,646,630, by Longaker and Sheynblat, describe an L-band pseudolite positioning system. The invention implies that a conventional GPS receiver can take full advantage of the additional information transmitted by the pseudolites. U.S. Pat. Nos. 5,708,440 and 5,686,924 by Trimble, et al., describe a pseudolite translator that transmits on unlicensed frequencies.
U.S. Pat. No. 6,101,178, Beal, describes a pseudolite system that operates on frequencies other than GPS L-bands and that combines CDMA and TDMA methods to address the near-far problem. The primary application of this patent is locating a cell phone, and it does not disclose a signal plan or a method for immediate resolution of carrier cycle ambiguities for precise positioning.
In all cases, these prior art pseudolite system designs do not provide sufficient methods to attain precise position solutions because they do not provide a dependable way to rapidly resolve carrier cycle ambiguities. The patents describe conventional techniques, such as L1 or L1-L2 Real-Time Kinematic (RTK) survey, which requires satellite motion. Though not discussed in their disclosures, these systems could, at best, use satellite motion for resolving integers and then back-out the solution for pseudolites, which could take several tens of minutes when fewer than four satellites are available. In the worst case, cycle ambiguity resolution may be impossible when there are only pseudolites and no satellites visible, and the mobile receiver is not moving or only moving small distances, as may be the situation for many applications. Therefore, such systems would only be able to provide several-meters of accuracy using code phase measurements.
Further, the Longaker, Sheynblat, and Trimble systems do not describe a signal plan different from the GPS specification, which is known to have limited dynamic range for close-range transmitters, and is regarded as the “near-far” problem. The GPS signal's C/A-code structure provides a signal-to-interference (S/I) ratio of 15.6 dB, which results in a maximum range ratio of about 6:1. Most real implementations exhibit a typical range ratio of more like 3:1 due to non-isotropic antenna gain patterns. A receiver inside the “near zone” of a pseudolite will be jammed from tracking satellite signals and other pseudolite signals by the pseudolite, while a receiver outside the “far zone” will not track the pseudolite signal at all. A system based on the Longaker, Sheynblat, or Trimble designs may have a very narrow intersection of usable space where all pseudolites and satellites can be received simultaneously. Further still, the L-band system described by Longaker and Sheynblat would require special government licensing to transmit on the GPS L-band frequencies.
U.S. Pat. No. 6,198,432, Janky, describes a method for assigning pseudolite PRN codes, based on the assumption that pseudolites ought to use only those from the set of 37 codes assigned by GPS ICD-200.
U.S. Pat. No. 6,336,076, Farley et al., U.S. Pat. No. 6,300,898, Schneider et al., and U.S. Pat. No. 6,031,487 Mickelson, describe pseudolite systems to aid GPS receivers in situations in which the GPS satellite signals are jammed.
U.S. Pat. No. 5,886,666, Schellenberg et al., describe an airborne pseudolite navigation system.
U.S. Pat. No. 5,629,855, Kyrtsos et al., and U.S. Pat. No. 5,757,314, Gounon et al., each describe a GPS-based positioning system that incorporates pseudolites but do not address how to implement a pseudolite system to address known fundamental differences between GPS satellites and pseudolites.
None of the previously disclosed patents describes a pseudolite system that can provide very high utility for a wide variety of applications. Each lacks one or more of the following:                1. Immediate availability of carrier-phase accurate position solutions        2. Full compatibility/non-interference with existing GPS signals        3. Significant range of operation (coverage)        4. License-free operation        
Multi-frequency GPS receivers have been disclosed by Bogensberger, et al., U.S. Pat. No. 6,016,121; Hanson, et al., U.S. Pat. No. 5,943,363; and Lennen, et al., U.S. Pat. No. 5,923,287 and U.S. Pat. No. 5,805,108.
U.S. Pat. No. 5,923,287, Lennen, discloses a combined GPS/GLONASS satellite receiver. Also, U.S. Pat. No. 5,805,108, Lennen, discloses a multi-frequency receiver that makes use of the GPS L3 signal. The claims in these two patents pertain to satellite systems only, and do not include differing pseudolite methods.
U.S. Pat. No. 5,943,363, Hanson et al., describes a multi-frequency spread-spectrum receiver design that reduces complexity by multiplexing carrier and code phase accumulation and carrier phase removal functions and by moving sum-of-product buffering into RAM.
U.S. Pat. No. 6,016,121, Bogensberger, describes a multiple frequency GPS receiver that is designed to be more efficient in power consumption and circuitry cost than conventional dual-frequency (L1-L2) GPS receivers. Bogensberger's patent claims regard methods to reduce power consumption and overall receiver circuitry for a two frequency (L1,L2) embodiment of the invention.
None of the aforementioned multi-frequency GPS receiver patents consider specific frequencies that a receiver might employ to function with pseudolites, nor how carrier cycle ambiguities might be resolved within a single sample period to achieve centimeter-level positioning. The aforementioned patents, employing only L1 and L2 frequencies (of GPS or GLONASS) would require several measurement samples over at least a few minutes of time while the satellites move in their orbits to attain cycle ambiguity resolution, similar to any conventional dual-frequency GPS/GLONASS receiver.
Multi-frequency positioning systems, not based on GPS, have been disclosed by Flood, et al., U.S. Pat. No. 5,563,612; Spence, et al., U.S. Pat. No. 4,283,726; Mosyakov, et al., U.S. Pat. No. 3,883,873; and Kramer, et al., U.S. Pat. No. 3,040,315.
U.S. Pat. No. 4,283,726, Spence et al., describes a basic distance measuring system that works by measurement of the phase of the beat frequency between two different carrier frequencies. The Spence invention, as disclosed, has many shortcomings as a general positioning technique: (1) The Spence invention requires a unique set of frequencies for every transmitter added to the system; (2) Transmitter clocks are not synchronized, likely to limit accuracy; (3) No data is sent from the transmitter to user, further limiting system flexibility.
U.S. Pat. No. 3,883,873, Mosyakov et al., describes a radio frequency positioning system that employs multiple frequencies and measurements of phases to determine position. The system distinguishes individual transmit stations by time slots. Within each time slot, a sequence of pulses of different frequencies is broadcast. A master transmit station broadcasts an additional carrier to synchronize the entire system. Phases of the multiple-frequency pulses are measured at a receiving station. The phases of the beat frequencies of the multiple-frequency carriers provide a measure of the range from the transmitter to the receiver, and these ranges can be used to determine the position of the receiver.
U.S. Pat. No. 5,563,612, Flood et al., describes a low-power, dual-frequency emergency position indicating radio beacon (EPIRB). The method of position determination for EPIRBs is performed by measuring the frequency Doppler shift of the transmission signal by a group of low-earth orbit or geostationary satellites (COSPAS/SARSAT and INMARSAT). Positioning methods and accuracies of EPIRB technology, particularly 121.5 MHz class B beacons that the Flood patent discloses, are significantly lower than GPS-based methods.
Fundamental differences between GPS and position sensing of EPIRBs via Search and Rescue Satellite-Aided Tracking (SARSAT) include: 1) GPS transmits from satellite to the user; SARSAT transmits from the user to the satellite. Because of this, GPS can support an unlimited number of active users, while SARSAT has a limited number of active users (users are only active during an emergency). 2) GPS signal structure features direct-sequence CDMA encoding and data that enables the user to attain meter-level and even centimeter-level range measurements which can be converted into meter- to centimeter-level position solutions; SARSAT data, sent by the user to the satellite, is reserved for user identification purposes.
U.S. Pat. No. 3,040,315, Kramer, describes a two-frequency system that measures range by the phase difference between the radiation field and the induction field of a low-frequency signal, while using the radiation field of the high-frequency signal as a reference. The Kramer design would not likely achieve the accuracy of a GPS-based system for several reasons, including system clock accuracy and synchronization, distortion at close range, diminishing observability at far range, and variation of user antenna pattern and orientation.
The proposed signal structures of the aforementioned positioning systems are significantly different from code-division spread spectrum methods employed by GPS and may restrict overall system capability, including number of transmitters recognized by a receiver, number of transmitters that can be deployed, range and accuracy of these systems.